1. Field of the Invention
The present invention relates generally to air spring assemblies for use in automotive vehicle suspensions. More particularly, the present invention relates to a method and apparatus for determining the force produced by an air spring assembly.
2. Disclosure Information
Gas suspensions have been popularly used for controlling vehicle attitude either alone or in combination with other suspension systems. For example, U.S. Pat. No. 4,834,418 discloses an electronically controlled gas suspension for controlling vehicle attitude. Air spring assemblies have also been used in conjunction with active suspension systems such as shown in U.S. Pat. No. 5,060,959, assigned to the assignee of the present invention, which shows the use of an air spring assembly in conjunction with an electric active suspension unit. In the '959 patent, the air spring assembly is used to carry the static load of the vehicle so that the active suspension component can respond to the road force inputs only. The suspension unit of the '959 patent is provided with means to measure the force produced by the air spring so as to apportion the total load between the air spring and the active suspension component. The '959 patent proposes the use of a pair of load cells to determine the load carrying forces generated by the air spring and the electric active suspension so that the fraction of the total load carried by each of the air spring and the electric unit can be determined. Using a load cell accurately measures the force produced by the air springs, but load cells add weight to a vehicle and can be expensive when adding a plurality of load cells to each corner unit of the vehicle. Therefore, it would be advantageous to eliminate at least one of the load cells and still determine the load carrying force produced by the air spring to apportion the load between the air spring assembly and the active suspension component. Furthermore, in more conventional suspension systems, it would be desirable to know the load carrying force produced by the air spring assembly so that proper switching of an adjustable suspension unit can be accomplished to improve ride and handling characteristics of the vehicle.
U.S. Pat. No. 4,651,838 proposes a system and method for load leveling of a vehicle which measures the force produced by the air spring assemblies without use of a load cell. The method and apparatus include inflating an inflatable member of the air spring with a compressed gas at a substantially constant flow rate and sensing the pressure of the gas within the member during inflation. The pressure sensing occurs at substantially even time intervals. From the pressure value, a pressure-time history for the inflation of the member is generated and from this pressure-time history, force produced by the air spring for a static vehicle can be determined. However, the system of the '838 patent is not a dynamic system, relying only upon the inflating of the pressure member for a static, non-moving vehicle. The system does not take into account chassis dynamics, such as the velocity of the chassis relative to the wheel and tire assembly or the direction of movement of the chassis relative to the wheel and tire assembly which, as will be shown below, have a distinct effect on the force produced by the air spring assembly.